A switched-mode power converter (also referred to as a “power converter”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a direct current (“dc”) input voltage into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling conduction periods of power switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon modifies a duty cycle of a power switch of the power converter. The duty cycle “D” is a ratio represented by a conduction period of a power switch to a switching period thereof. Thus, if a power switch conducts for half of the switching period, the duty cycle for the power switch would be 0.5 (or 50 percent). Additionally, as the voltage or the current for systems, such as a microprocessor powered by the power converter, dynamically change (e.g., as a computational load on the microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the power switches therein to maintain an output characteristic such as an output voltage at a desired value.
In a server or other high-end power supply applications, a microcontroller is typically used in connection with the primary side of the power train of the power converter to handle higher-level power management tasks. Most present-generation microcontrollers operate from a 3.3 volt (“V”) bias voltage source, and can consume up to 100 milliamps (“mA”) or more of bias current during an operation thereof. To provide a 3.3 V bias voltage source for the microcontroller, a dissipative power supply referred to as a linear regulator is typically coupled to a higher input voltage source such as a 12 V bias voltage source to produce the 3.3 V bias voltage for the microcontroller. In many power supply designs, the input voltage to the linear regulator is produced by an auxiliary power converter that provides supply voltages for housekeeping needs including both primary- and secondary-side housekeeping needs. For primary-side housekeeping needs, a 12-14 V bias voltage source is typically provided for a pulse-width modulation (“PWM”) control integrated circuit (“IC”) and for a driver IC to drive primary-side power switches. A linear regulator can directly reduce the 12-14 V bias voltage to the 3.3 V bias voltage. A drawback of a linear regulator, however, is its power loss, which can be as large as one watt (“W”), and is significant due to the large voltage drop produced by the linear regulator. In a high-efficiency power supply design, the power loss produced by such a linear regulator is an important loss component in view of a typical efficiency target at light load, as well as a need for careful thermal management of the linear regulator power dissipation.
In a conventional solution to convert 12-14 V down to 3.3 V, the linear regulator is replaced with a power converter such as a small dc-dc power converter to provide 3.3 V, or to provide a 5 V bias voltage followed by a low-dropout linear regulator. The dc-dc power converter typically provides high efficiency, which can be greater than 90%, but the cost and component count of the dc-dc power converter as well as the printed circuit board area that it occupies can be significant drawbacks of such a design.
Cost and efficiency compromises provided by conventional approaches to providing an auxiliary bias voltage in a power converter have become obstacles in the high-volume, competitive marketplaces now being served by such designs. Thus, despite continued size and cost reductions of components associated with power conversion, no satisfactory strategy has emerged to resolve the issues associated with providing a small, efficient, and low-cost bias voltage in a power converter for an internal housekeeping function. Accordingly, what is needed in the art is a circuit and related method to produce an internal bias voltage in a power converter that avoids the aforementioned obstacles, particularly for high-volume, low-cost manufacture of power adapters and other power supplies employing the same.